Radiation detection

ABSTRACT

An instrument for detecting radiation is provided, which comprises an inner core housing a neutron detector, and an outer core comprising a neutron-moderating material, the instrument further including at least one elongate thermal neutron guide located within the outer core and having an inner end that terminates proximal to the neutron detector. In use, the elongate thermal neutron guide channels thermal neutrons towards the neutron detector. Also provided is a method for using said instrument.

This invention concerns instruments and methods for measuring neutronradiation, in particular for measuring dose equivalent rates fromneutrons.

Accurate measurement of neutron dose rates is important in ensuring thatenvironments intended for operations involving personnel access areaccurately surveyed and/or that personnel operating in environmentswhere significant radiation exists receive a dose suitably below thelimit permitted. It is also important in ensuring that the dose rates inpublic access areas are below the permitted limits.

In most cases the field consists of neutrons with a range of energies.The spread of this distribution and the proportions of the dose in eachpart are significant because of the differing level of biological hazardthey present. Thus, there is a need in the art for a device that canaccurately detect neutrons with low (<0.4 eV), intermediate (0.4 eV to10 eV) and high (10 keV to 20 MeV) energies.

Prior art instruments are limited in terms of the range of neutronenergies they can monitor accurately—particular problems occur at thehigh energy range as such particles can have a high probability ofpassing straight through the device without being detected. This problemis illustrated by the devices described in U.S. Pat. No. 4,588,898. Inmore detail, whilst the use of polyethylene helps to moderate fast andintermediate energy neutrons, the volume of polyethylene necessary tomoderate high energy neutrons efficiently is prohibitive in terms ofweight and size. Conversely, the use of excessive polyethylene preventsefficient detection of thermal (i.e. low energy) neutrons. Such devices,therefore, detect neutrons in parts of the energy range with lowefficiency.

One means for addressing the above-mentioned problem is to employ adevice that detects neutrons over a limited energy range, and then toapply a mathematical “weighting formula” to the results to obtain a“total dose equivalent”.

Such mathematical weighting is, however, fraught with inaccuracies, asit avoids direct measurement of neutrons across a wide range of energylevels.

A second problem associated with prior art instruments is that thesensitivity may vary significantly according to the energy range ofinterest. Thus, if the device in question is calibrated using highenergy neutrons, the reading in the presence of lower energy neutronscan be erroneous. Likewise, if it were possible to perform calibrationusing a low energy source, readings for higher energy neutrons would beincorrect. Whilst the use of supplemental spectroscopic measurements canbe employed to establish the energy distribution of the neutronsdetected and so apply a correction factor, this is time consuming,expensive and impractical. It also relies on the fields in which theinstrument is used not varying from location to location, or within alocation.

The above-mentioned inaccuracies have been tolerated in the past—it wasthought the worst deviations occurred in parts of the neutron energyrange that were not of great importance or where little dose equivalentwas received. More recently, however, it has been realised that therelative risk factor for neutrons is greater than previously accepted(the maximum quality factor for neutrons was increased from 20 to 30relative to that for photons). As a result, increased sensitivity withless deviation throughout a wide energy range is required.

As a further complication, the definitions used in radiation protectionare periodically updated. When this causes changes to the quality factoror radiation weighting factor for neutrons, the relative importance ofdifferent parts of neutron energy range changes. The nature of manyprior art instruments means that such changes would require wide-scalereconfiguration of the device, even necessitating hardware replacement,or else acceptance of large deviations. A more versatile instrument isthus required.

EP943106 describes a neutron-detecting device that demonstrates someimprovements over earlier detection devices. In more detail, thedescribed device is based on a central, ³He core detector, whichoperates in combination with six photodiodes located at a shallow depthbelow the surface of a moderating polyethylene sphere as neutrondetectors. The central detector is located within a polyethylene shell,which is encased by a boron-loaded rubber shell (Flex/Boron). Outsidethe boron-loaded rubber is a further polyethylene shell. In use, theouter polyethylene shell moderates fast neutrons, the boron-loaded layerattenuates low-energy neutrons incident on the instrument, and the innerpolyethylene shell moderates further the neutrons that penetrate theboron-loaded layer. The EP9431060 device makes it possible to constrainthe H′*(10) response to within a factor of 2 of a presumed calibration,which represents an improvement over pre-existing neutron area surveyinstruments (Bartlett et al, 1997).

Whilst the EP943106 device offers some advantages in terms of energydependence of response, it is expensive to construct compared with priordesigns because of the requirement for seven detectors. In addition, therequirement for both high and low voltage power supplies complicates theinstrument further and hence adds to the price. A further problemassociated with the EP943106 device is that it is relatively heavy—thisis a particular problem in the context of hand-held devices, the use ofwhich requires an operator to hold the device at arms-length to avoidinteractions caused by the operator's body.

Thus, there is a need for a more practical neutron survey instrumentthat addresses one or more of the above problems.

The present invention solves one or more of the above problems.

According to a first aspect of the present invention, there is providedan instrument for detecting radiation, wherein the instrument comprises:i) an inner core comprising a neutron detector; ii) an outer corecomprising a neutron-moderating material, said outer core having anexternal surface; and iii) at least one elongate thermal neutron guidelocated in the outer core, said elongate guide having an inner end andan outer end, wherein:

(a) the thermal neutron guide extends in a direction from the externalsurface of the outer core to the neutron detector and, in use, channelsthermal neutrons towards the neutron detector, and

(b) the inner end of the thermal neutron guide is proximal to theneutron detector.

In one embodiment, the instrument, more specifically the inner coreneutron detector, is sensitive primarily to thermal (low energy)neutrons and obviates the need for multiple detectors. In the context ofthe present invention, low energy (ie. thermal) neutrons means thosewith an energy of less than 0.4 eV. Said thermal neutrons preferablyhave a mean energy (at room temperature) of less than 100 meV, or lessthan 50 meV, or less than 30 meV, for example in the region of 25.3 meV.

In use, the instrument reduces overestimates of intermediate energyneutrons (0.4 eV to 100 keV), which are characteristic of other singledetector survey instruments.

During operation, the instrument is preferably held away from theoperator at arms' length. To facilitate this, the instrument has amaximum weight that is less than 10 kg, for example less than 8 kg, orless than 6 kg.

In one embodiment, the neutron-moderating material is a solid materialthat comprises one or more hydrogen-containing material, such as one ormore plastic material, e.g. polyethylene or another hydrogen-containingor hydrocarbon polymer. The use of a material with an average density ofbetween 0.6 and 1.5 g/cm³ is preferred, more preferably between 0.7 and1.2 g/cm³, even more preferably between 0.8 and 1.15 g/cm³, and mostpreferably between 0.90 and 1.00 g/cm³.

The neutron-moderating material may comprise one or more differentmaterials. In one embodiment, one or more of said different materialsmay be arranged in corresponding layer(s), preferably a discretelayer(s) around the inner core. In another embodiment, two or more ofsaid different materials may form composites, mixtures and/or amalgams,which may be arranged as one or more layer(s), preferably a discretelayer(s) around the inner core. The one or more different materialspreferably have a high thermal neutron capture cross-section. In oneembodiment, each layer has a thickness in a direction from the innercore to the external surface of the outer core. By way of example, thedirection is the direction that provides the shortest distance from theinner core to the external surface of the outer core. In this regard, inan embodiment where the outer core is substantially a spherical shape,the direction would be a substantially radial direction.

The outer core may contain a neutron-attenuating material, such as boronor a boron-containing material, cadmium or a cadmium-containingmaterial, lithium or a lithium-containing material, or composites,mixtures and/or amalgams thereof. Where the neutron-attenuating materialcomprises boron or boron-containing material, the boron may be naturalor enriched in a boron isotope such as boron-10. If the materialcomprise lithium or a lithium-containing material, the lithium may beeither natural or enriched in a lithium isotope such as lithium-6. Theneutron-attenuating material may be provided in powder form, as a metallayer(s), or in a matrix such as a plastic matrix. In one embodiment,the neutron-attenuating material forms one or more layers (preferably adiscrete layer(s)) within the outer core. In one embodiment, theneutron-attenuating material may form a layer that defines an innersurface of the outer core. Alternatively, the neutron-attenuatingmaterial may be sandwiched between neutron-moderating material(s)located within the outer core—the sandwiching neutron-moderatingmaterial(s) may be present as layers.

Preferably the outer core substantially surrounds the inner core. Theouter core may contact the inner core.

The outer core may be provided with a carrying handle and/or anexternally mounted electronic processing means. It may also haveprotruding feet. The processing means may alternatively be positionedaway from the device. In use, the processing means is in communicationwith the device. Suitable processing means includes one or more of hardwiring, optical, radio or other means.

The inner core may comprise or consist of one or more neutron detectors.The detector(s) may be selected from one or more of the ³He type; theBF₃ type; and a ⁶Li and ⁷Li converter pair, scintillators and photomultiplier tubes, or a material from which delayed beta decays can bedetected, e.g. silver. Preferably, the instrument comprises or consistsof a single neutron-sensitive detector.

In one embodiment, the inner core is spherical. Preferably the outercore is shaped or dimensioned to provide a substantially even thicknessof material around the inner core. The outer core may be spherical.Alternatively, the outer core may be cylindrical with hemisphericalends, in which case the inner core preferably has a corresponding,though smaller shape.

The device may comprise one or more thermal neutron guide(s). Preferablythe device comprises at least 2 or 4, or at least 6 or 8, or at least 10or 12 thermal neutron guides. In one embodiment, the device comprises 14thermal neutron guides.

Preferably, the thermal neutron guide(s) are arranged symmetricallyaround the inner core. In one embodiment, the guide(s) extend in agenerally radial direction from the inner core or detector to theexternal surface of the outer core.

The thermal neutron guide(s) may extend all the way from the externalsurface of the outer core to the inner core/detector. Alternatively, thethermal neutron guide(s) may terminate short of the inner core and/orshort of the external surface of the outer core. In a preferredembodiment, the thermal neutron guide(s) contact with the inner core,preferably with the central detector.

In one embodiment, the thermal neutron guide(s) are structurallydistinct from (and preferably free from) neutron-moderating and/orneutron-attenuating material.

In one embodiment, the thermal neutron guide(s) extend in a directionaway from the external surface of the outer core to the innercore/detector. Preferably, the thermal neutron guide(s) provide a singleline-of-sight for channelling thermal neutrons from outside of thedevice to the inner core. In one embodiment, the guide(s) are elongateand straight. It is preferred that the thermal neutron guide(s) extendin a direction that provides the shortest distance from the inner coreto the external surface of the outer core. By way of example, in anembodiment where the outer core is substantially a spherical shape, thedirection would be a substantially radial direction.

The thermal neutron guide(s) may comprise or consist of a solidmaterial, preferably a metal such as aluminium or lead, most preferablyaluminium.

Alternatively, the thermal neutron guide(s) may comprise or consist of afluid material, preferably air. In this embodiment, the guide(s) arepreferably provided by bore holes in the neutron-moderating and/orneutron-attenuating material. Alternatively, the thermal neutronguide(s) may comprise of a vacuum or a partial vacuum.

The cross-sectional area of the thermal neutron guide(s) may besubstantially constant along their whole length. Alternatively, thecross-sectional area of the thermal neutron guide(s) may vary alongtheir length. In one embodiment, the cross-sectional area increases inthe direction away from the inner core towards the external surface ofthe outer core. In a preferred embodiment, the cross-sectional area of aportion of the guide(s) located towards the external surface of theouter core may be greater than the cross-section of a portion of theguide(s) located towards the inner core.

In one embodiment, the thermal neutron guide(s) extends in a directiontowards the inner core such that the inner end of the guide(s) is inclose proximity to the detector. By way of example, the inner end of theguide(s) may terminate at a position such that the distance between theguide end and the inner detector is less than 20 mm, less than 15 mm,less than 10 mm, less than 8 mm, or less than 5 mm. Alternatively, theguide(s) may extend all of the way to the detector (ie. abut with thedetector) at the heart of the instrument. In this regard, if there is asignificant amount of moderating material between the inner end of theguide and the detector, then the response of the instrument will beadversely affected: the guide will not be able to effectively raise aresponse to thermal neutrons. This problem is observed with severalprior art devices, such as the detector described in DE19627264C1, andis addressed and solved by the present invention.

The elongate thermal neutron guide(s) may terminate (at or near theexternal surface of the outer core) in a flat or substantially flatouter end. Alternatively, the outer end may terminate in a concave axialcross-section shape, such as an inverted point, an inverted dome, asemi-circle, a “V”-shape or a “U”-shape (as shown, for example, in someof the accompanying Figures). Thus, the outer end of a guide may beshaped to modify the direction dependence of response for low energyneutrons. For example, with a flat guide end (or with a guide end thatis flush with the outer surface of the instrument), the instrumenttypically responds preferentially to neutrons that are incident planeparallel along the axis of a particular guide. Alternatively, provisionof a concave outer guide end modifies the direction dependence ofresponse of the instrument, which is preferred. By way of example, theconcave end of a guide may provide an average concave radius ofapproximately or up to 20 mm, 15 mm or 10 mm. A concave guide feature ismore than just a simple neutron channel, since it is so designed toavoid the instrument from over-responding to neutrons incident fromspecific directions. By way of example, an instrument may comprise 2, 4,6 or 8 such concave guides. The geographic arrangement/positioning ofconcave guides versus non-concave guides (e.g. conventional guides) inan instrument may be random. Alternatively, concave guides may bearranged into ‘pairs’, such that the two members of each pair arelocated on substantially opposite sides of the detector (e.g. therespective concave guide ends of a pair may be diametrically opposed).In one embodiment, all of the instrument guide(s) are concave guide(s).

The guide(s) may include a protective end to prevent debris fromentering the device from the external surroundings. The protective endmay be in the form of a plug. By way of example, a plug may be formedfrom the same material as the moderator material or from a materialchosen to help thermal neutrons enter the thermal neutron guide(s), e.g.a metal. In one embodiment, the protective end material per se does notsubstantially moderate or attenuate neutrons, especially low energyneutrons. Alternatively, the protective end may be a continuation of acomponent of the outer core material.

In one embodiment the instrument is capable of efficiently detectingneutrons in the energy range of 0.1 meV to 20 MeV (or higher), or in theenergy range of 0.5 meV to 15 MeV, or in the energy range of 1 meV to 10MeV,

In a preferred embodiment, the instrument can detect neutrons in asubstantially non-directionally sensitive manner. In one embodiment,where the electronics of the device are detached, the response does notvary for any angle of incidence. In another embodiment, where theelectronics are attached to the moderator, the response does not varyappreciably for any angle of incidence in the plane perpendicular to theaxis through the electronics and moderator. In that embodiment, theresponse does not vary appreciably for an angle in a ±150° arc from theaxis through the electronics and moderators, where 0° is defined asincidence from opposite the electronics.

According to a second aspect of the invention, there is provided amethod of detecting radiation using an instrument according to the firstaspect of the invention, wherein said method comprises contacting theinstrument with neutrons, generating one or more signals followingcontact between the neutrons and neutron detector, and detecting saidone or more signals.

According to a further aspect of the invention, there is provided amethod for enhancing the detection of thermal neutrons incident on aninstrument comprising a neutron detector, said method comprising thepreferential channelling of thermal neutrons along at least one elongatethermal neutron guide having an inner end that terminates proximal tothe detector, followed by detection thereof by the neutron detector.

A further aspect of the invention provides use of an elongate thermalneutron guide(s) in an instrument for detecting thermal neutrons,wherein the guide(s) terminate at a position proximal to the detector,and wherein, in use, the thermal neutron guide(s) channel thermalneutrons towards a neutron detector. The thermal neutron guide is afeature located in the moderator/absorber designed to:

-   -   effectively channel neutrons to the centre of the device    -   modify the efficiency of the channelling according to the        direction and energy of the neutron    -   ensure passage through the absorbing layer and all the way to        the central detector

The guide may have variable cross-sectional area to ensure that theinstrument does not preferentially detect neutrons within a particularenergy range.

Various embodiments of the invention will now be described withreference to the drawings in which:

FIG. 1 shows a cross-sectional representation of one embodiment of theinstrument of the present invention. The thermal neutron guide(s) areshown extending from the inner core (detector) to the external surfaceof the outer core. The outer core comprises a neutron-attenuating layersandwiched between neutron-moderating layers.

FIG. 2 shows the fluence response for 1.5 cm cross-sectional diameteraluminium, copper and lead rods extending from an inner detector to theouter surface of the device. The data are for a boron-loadedpolyethylene attenuating layer with inner and outer radii of 3.7 and 4.9cm.

FIG. 3 shows the H*(10) response data for 15 mm and 6 mm cross-sectionaldiameter rods that extend to the outer surface of the moderator.

FIG. 4 shows a vertical cross-sectional representation of a deviceaccording to one embodiment of the present invention showing the neutronguide(s), inner core, and the polyethylene-moderating material andpolyethene-attenuating material of the outer core.

FIG. 5 shows the fluence response data for aluminium rods that reach theexternal surface of the outer core of the device or stop 5 mm beneaththe external surface. The data are for aluminium rods with a diameter of6 mm.

FIG. 6 shows the H*(10) response data for aluminium rods that reach theexternal surface of the outer core of the device or stop 5 mm beneaththe external surface.

FIG. 7 shows the effect of varying the location of theneutron-attenuating boron-loaded layer for 6 mm cross-sectional diameteraluminium guide(s) that extend from the central detector to a depth of 5mm from the external surface of the outer core of the device.

FIG. 8 shows the effect of changing both the guide cross-sectionaldiameter and the thickness of CH₂ covering the ends of the guide(s),i.e. between the ends of the guide(s) and the external surface of theouter core.

FIG. 9 shows the H*(10) response of the exemplified design showing datafrom the optimization calculations and for more energies. Three energydistributions have also been used: thermal, ²⁵²Cf and ²⁴¹Am—Be. Theseare not plotted as part of the curve.

FIG. 10 shows the H*(10) response to unidirectional and isotropicsources. The unidirectional source is directed along the axis of two ofthe guide(s). These data are for neutron guides with flat outer ends.

FIG. 11 shows the H*(10) response of a device with neutron guides thatpenetrate the neutron absorbing layer, but which do not carry on furtherto reach the detector. The H*(10) response of a device which has guidesthat channel neutrons all the way to the central detector is alsodepicted for contrast. The bores which do not extend all of the way tothe central detector are seen to fail to guide thermal and low energyneutrons effectively for detection.

FIG. 12 shows the ambient dose equivalent response characteristics of adevice according to the present invention, compared with those for theLeake and EP943106 devices.

FIG. 13 shows a device with hemispherical inserts into the end of eachguide which are intended to modify the directional dependence ofresponse at low energies. The plugs at the end of the guide may be madefrom the same material as the outer moderator or from a material chosento ease construction or modify the response characteristics.

FIG. 14 shows the ambient dose equivalent response characteristics of adevice that has 7 mm radius hemispherical inserts into the end of theguide. The device has been modelled for plane parallel and isotropicneutron fields. The results should be contrasted with those presented asFIG. 10.

FIG. 15 shows a cylindrical device with a single neutron guide.

FIG. 16 shows a “bullet-shaped” device with two neutron guides in a cutthrough the middle of the device. Additional guides project out of andinto the page, making a total of four guides.

FIG. 17 shows a cubical device with four neutron guides in a slicethrough the middle. Additional guides project out of and into the page,making a total of six guides

FIG. 18 shows a cubical device with a complex pattern of guides. Theseinclude guides that extend from the outside to the attenuating layer,from the middle of the device to the attenuating layer and guides thatpenetrate the attenuating layer.

EXAMPLE 1

A device is provided with six thermal neutron guides spaced orthogonallyaround the inner core (four shown) (see FIG. 1). The neutron-moderatinglayers of the outer core comprise a CH₂ based polymeric material such aspolyethylene. The neutron-attenuating layer of the outer core comprisesa boron-containing material, such as boron-loaded polyethylene.

The inner core comprises a neutron proportional counter. Neutronsincident upon the counter may produce alpha particles via the ¹⁰B(n, α)reaction in BF₃, or protons and tritons via the ³He(n, p)T reaction,which cause ionisation. Pulses are detected and give rise to a signalwhich passes from the detector to the electronics. The detector thusprovides a signal indicative of the number of events in the detector.The signal from the detector may be monitored and recorded for arepresentative time period.

Test Parameters

Investigations were performed on the embodiment shown in FIG. 1, thedevice having the following characteristics:

TABLE 1 Radii of shells. Layer Inner radius (cm) Outer radius (cm)Detector case 1.60 1.65 Inner CH₂ 1.65 2.7 Boron-loaded layer 3.7 4.9Outer CH₂ 3.9 10.48

The central detector used in the design retained the dimensions of theCentronic Limited SP9 spherical proportional counter, which uses ³He asthe fill gas.

EXAMPLE 2

The device according to Example 1 was prepared with a range of differenttypes of neutron guide. Once assembled, the guides have an approximatecross-sectional diameter of 1.5 cm, and extend to the external surfaceof the outer core. Three very distinct metals were selected forinvestigation, namely, copper, aluminium and lead. These metals wereselected for the following reasons:

-   -   Copper: intermediate density, relatively high (n, 2n)        cross-section (˜0.6 b), which offers significant potential for        increasing the response to high energy neutrons    -   Aluminium: light, very transparent to thermal neutrons, and low        (n, 2n) cross-section (˜0.035 b)    -   Lead: very transparent to thermal neutrons, with a high (n, 2n)        cross-section (˜2 b)

For neutron energies of 1 MeV and above the choice of metal is notsignificant (FIGS. 2 and 3): the use of lead increases the response at20 MeV over that for aluminium or copper, but only by about 20%. Forlower energies (e.g. thermal neutrons), the response with the lead rodsis 34 times higher than that for copper rods, whereas the response foraluminium rods is 250 times higher than that for copper rods. The mainreason for this sensitivity is that the (n, γ) cross-sections varysignificantly. When the density is taken into account, the capturecross-sections for lead and copper are four and 66 times that ofaluminium respectively. Thus, whilst any one of these materials may besuccessfully employed as a neutron guide for the purpose of the presentinvention, the use of aluminium is preferred as it is highly transparentto thermal neutrons. Alternatively, other easily-machined metals may beused if they have a low atomic mass and no strong neutron capturereaction cross-sections.

When either lead or aluminium is used in 1.5 cm diameter guides, theH*(10) response to thermal and intermediate energy neutrons is high(FIG. 3). The difference in the lead and aluminium data may be connectedto the stronger elastic scattering in lead: more neutrons will bescattered out of the guide. It is not connected to the radiative capturecross-sections for thermal neutrons because those are 0.231 b foraluminium and 0.174 b for lead. For fast neutrons, the (n, γ)cross-section for lead is much higher than that for aluminium, with alot of resonances, but for that energy range the response should bedominated by neutrons that are moderated in the CH₂ layer.

The response to thermal neutrons for aluminium and lead simply indicatesthat the rods are efficient at getting thermal and intermediate energyneutrons through the boron-loaded polyethylene. Aluminium also offersconsiderable mass savings because of its low density:

EXAMPLE 3

The device described and employed in Example 2 includes neutron guidesthat extend to the external surface of the outer core of the device. Thepresent Example describes and employs a device in which the thermalneutron guides do not reach the external surface of the outer core. Inthis Example, guides that have 5 mm of polyethylene between their endand the external surface of the outer core are employed (FIG. 4).

The results for this new arrangement are contrasted with those forguides that reach the surface of the device in FIGS. 5 and 6. These datashow that for all energies of 10 keV and above, the difference in theresponse is not resolved by the calculations. This is because suchenergetic neutrons are not strongly moderated or attenuated in theabove-described first 5 mm of the outer core.

The H*(10) response (FIG. 7) is seen to be markedly improved by thismodification, with the thermal neutron response now being slightly lowerthan that for fast neutrons.

EXAMPLE 4

The present Example demonstrates how the fast neutron response of adevice according to the present invention may be modified according tothe location of the neutron-attenuating layer, in this case aboron-loaded polyethene layer.

In each case, the thickness of the boron-loaded polyethylene layer wasnot varied: it remained 1.2 mm. Three different positions for theneutron-attenuating layer were investigated: inner and outer radii of 32and 44 mm; 37 and 49 mm; 42 and 54 mm. These 5 mm increments do notproduce a very significant effect on the response for energies of 10 eVand below, which is simply an indication that the response to neutronswith those energies is dominated by neutrons that travel along theguides (FIG. 7). For higher energies the response increases as theattenuating layer is moved further from the centre of the instrumentbecause the attenuating layer becomes less effective and the moderationprovided by the inner core becomes more effective.

EXAMPLE 5

Referring to the device(s) described in Example 3, two changes were madesimultaneously to the device described in the present Example. First,the cross-sectional diameter of the guides was reduced to 5 mm from 6mm. Secondly, the ends of the guides were extended by 1 mm, so that theyterminated 4 mm as opposed to 5 mm from the external surface of theouter core of the device. The changes were made at the same time becausethey were intended to be complimentary: reducing the diameter of theguide will lower the thermal neutron response, but reducing the CH₂covering the end of the guide should raise the thermal neutron response.

The effect of these changes is not dramatic. No change in the responsefor neutrons incident with energies of 10 keV or above is detected (FIG.8), which is an indication that the response to those energies is notstrongly dependent on the guides. There is also no significantdifference for thermal neutrons, which probably shows that the narrowingof the guides offsets the reduction in polyethylene that must betraversed to reach the guides. These are the least penetrating neutrons,so they will enter the guide most strongly at the end.

For neutrons with energies from 0.1 eV to 5 keV, the two changestogether cause a reduction in the response. Of the data calculated, themost significant difference is at 10 eV, for which the changes cause theresponse to fall by almost 40%. There is an improvement in the responseat 5 keV, which reduces the magnitude of the potential overestimate atthat energy.

When more source energies are used, the energy dependence of response isseen in more detail (FIG. 9). It is then seen that the minima of theH*(10) response are for energies around 20 eV and 200 keV and themaximum response is for 5 keV.

Three energy distributions were also used as sources: a 300 KMaxwell-Boltzmann distribution (thermal), and ²⁵²Cf and ²⁴¹Am—Beradionuclide sources (ISO, 2001). The line does not connect these sinceit is simply intended to join up the monoenergetic response data. It isseen that the response to a thermal neutron energy distribution ishigher than would be expected from the monoenergetic data. To calculatethe thermal neutron ambient dose equivalent response, a value of 11.4pSv cm² has been used for the fluence to dose equivalent conversioncoefficient. This differs from the 25.3 meV value tabulated by ICRU andICRP of 10.6 pSv cm² which is only applicable for a monoenergeticneutron field. The calculated response for the energy distribution issignificantly higher than that for the monoenergetic field. The responseto an isotropic source in MCNP is found to be the same as that for aunidirectional source for fast neutrons (FIG. 10).

EXAMPLE 6

A preferred embodiment of the current invention has the specificationsshown in Table 2. This design would have a total moderator mass of 4.52kg.

TABLE 2 Dimensional and material specifications of the design and totalmass calculation for Example 6 Inner Outer Density Material radius (cm)radius (cm) (g cm⁻³) Mass (kg) ³He 0 1.6 2.4 10⁻⁴ 0.00 Steel 1.6 1.657.86 0.01 Inner CH₂ 1.65 3.2 0.93 0.11 Boron- 3.2 4.4 0.98 0.21 loadedCH₂ Aluminium 1.65 10.0 2.7  0.04 guides^(†) Outer CH2 4.4 10.48 0.934.15 Total 4.52 ^(†)6 mm cross-sectional diameter

On the assumption that the addition of electronics and batteries wouldadd no more than 1 kg to the total mass, the instrument would then besignificantly lighter than the other commercially available designs: theNMS017 (Leake) has a mass of 6.2 kg; the SWENDI-II weighs 13.4 kg; theWedholm Medical 2222D 10.5 kg; the Berthold LB6411 9.0 kg. This alonewould be an attractive feature of the design, because a device thatweighs less than 6 kg would be relatively easy to use in the workplace.

Users will not only be attracted solely by lightness of a surveyinstrument of the present invention, but will also be interested in itsdosimetric performance: there are plenty of very light instrumentsavailable which do not have acceptable dose equivalent responsecharacteristics. For example, the NMS017 has a total mass of only 2 kg,since it uses a 5″ diameter moderating sphere (6.35 cm radius), but itsH*(10) response to fast neutrons is more than two orders of magnitudelower than its response to thermal neutrons. Its H*(10) response toregions in the keV energy range is an order of magnitude higher than itsresponse to thermal neutrons, so the response varies by more than afactor of 1000 in the energy range up to 20 MeV.

The most directly comparable of the widely used neutron surveyinstruments is the Leake design, since it is the lightest. It does notperform as well as some of the others dosimetrically, especially athigher energies, but it is the most widely used in the UK. When thecomparison is made with a device of the present invention (FIG. 11) itis seen that the overestimate in the keV energy range is substantiallyreduced. The under-response to thermal neutrons is also eliminated, andthe fast neutron response is slightly better. The energy dependence ofresponse characteristics is clearly superior to those of the Leake.

Perhaps the best response characteristics of those previously publishedare those of the NRPB/BNFL design (Bartlett et al, 1997) with sevendetectors, and a device mass of approximately 10 kg (FIG. 11). However,when compared to with a device of the present invention, the NRPB/BNFLdevice response characteristics are, on balance, poorer. In particular,the dip in the response at 100 keV and the peak at 5-10 MeV are reducedor removed by the use of guides in accordance with the presentinvention. These latter two features are highly significant in theworkplace. The NRPB/BNFL device was also substantially heavier, whichwould cause significant operational disadvantages.

EXAMPLE 7

A preferred embodiment of the current invention has the specificationsshown in Table 3. This design would have a total moderator mass of about5 kg. This differs from Example 6 in that the guides are filled withair/vacuum instead of aluminium and they change radius at theattenuating layer: the guides are thinner through the inner moderatinglayer because their function is simply to channel thermalized neutrons,whereas in the outer moderating layer, their function is to channel andpreferentially accept thermal neutrons. The boron-loaded attenuatinglayer in this example is located further from the detector, which aidsthe response to high-energy neutrons. The location of the boron-loadedattenuator in Example 6 would be preferred for fields which do notcontain a significant component of fluence from high-energy neutrons.

TABLE 3 Dimensional and material specifications of the design and totalmass calculation for Example 7 Inner Outer Density Material radius (cm)radius (cm) (g cm⁻³) Mass (kg) ³He 0 1.6 2.4 10⁻⁴ 0.00 Steel 1.6 1.657.86 0.01 Inner CH₂ 1.65 5.0 0.92 0.40 Air guide (inner)^(†) 1.65 5.00.0012 0.00 Boron- 5.0 6.25 0.98 0.80 loaded CH₂ Air guide (outer)^(‡)5.0 10.25 0.0012 0.00 Outer CH2 6.25 11.00 0.92 3.74 Total 4.95 ^(†)7 mmcross-sectional diameter ^(‡)1.4 mm cross-sectional diameter

EXAMPLE 8

The preferred embodiment of the device utilizes the parameters specifiedfor Example 7 in Table 3, with the addition of hemispherical inserts tothe end of each guide as shown in FIG. 13. The radius of the outersection of the neutron guide in each case is 0.7 cm and the radius ofthe sphere that forms the insert is 0.7 cm. The centre of the sphere islocated 0.75 cm below the outer surface of the moderator.

In this embodiment the boron-loaded attenuating layer could be locatedas given for Examples 6 or 7 in Tables 2 or 3 respectively.

REFERENCES

-   Bartlett, D T, Tanner R J and Jones D G (1997). A new design of    neutron dose equivalent survey instrument. Radiat Prot Dosim, 74    (4), 267-271.-   Briesmeister J F (Ed) (2000). MCNP—a general Monte Carlo n-particle    transport code, Version 4C. Report No. LA-13709-M. Los Alamos: LANL.-   International Organization for Standardization (2001 a). Reference    neutron radiations—Part 1: characteristics and methods of    production. ISO 8529-1:2001 (E). Geneva: ISO.-   Leake J W (1965). A spherical dose equivalent neutron detector. Nucl    Instrum Meth, 45, 151-156.-   Leake J W (1968). An improved spherical dose equivalent neutron    detector. Nucl Instrum Meth, 63, 329-332.-   Leake J W (1999). The effect of ICRP (74) on the response of neutron    monitors. Nucl Instrum Meth, A421, 365-367.-   Tanner, R J, Molinos, C, Roberts, N J, Bartlett, D T, Hager, L G,    Jones, L N, Taylor, G C and Thomas, D J (2006). Practical    implications of neutron survey instrument performance. HPA-RPD-016    (Chilton: HPA).

The invention claimed is:
 1. An instrument for detecting radiation, theinstrument comprising: i) an inner core comprising a neutron detector;ii) an outer core comprising a neutron-moderating material, said outercore having an external surface; and iii) a plurality of elongatethermal neutron guides having an inner end and an outer end, whereinsaid plurality of elongate thermal neutron guides are arranged in asubstantially symmetrical pattern in the outer core, and wherein: (a)the plurality of elongate thermal neutron guides extend in a directionfrom the external surface of the outer core to the neutron detector and,in use, channel thermal neutrons towards the neutron detector, (b) theinner ends of the plurality of elongate thermal neutron guides areproximal to the neutron detector, and (c) the plurality of elongatethermal neutron guides are configured for channeling thermal neutronsfrom outside of the device to the neutron detector along the thermalneutron guides from the outer ends to the inner ends, and then to theneutron detector.
 2. An instrument according to claim 1, wherein theneutron-moderating material comprises a hydrogen-containing material. 3.An instrument according to claim 1, wherein the inner ends of theplurality of elongate thermal neutron guides and the neutron detectorare separated by a maximum distance of less than 20 mm or less than 15mm.
 4. An instrument according to claim 3, wherein the outer corefurther comprises one or more different material arranged as a layerwithin the outer core.
 5. An instrument according to claim 4, whereinsaid one or more different material is a neutron-attenuating material.6. An instrument according to claim 5, wherein: (i) theneutron-attenuating material substantially surrounds the inner core,and/or (ii) the plurality of elongate thermal neutron guides aresubstantially free of neutron-attenuating material, and/or (iii) theinner core consists of a neutron detector.
 7. An instrument according toclaim 4, wherein said one or more different material is boron or aboron-containing material.
 8. An instrument according to claim 4,wherein said one or more different material is sandwiched between firstand second neutron-moderating material layers.
 9. An instrumentaccording to claim 4, wherein said one or more different materialsubstantially surrounds the inner core.
 10. An instrument according toclaim 1, wherein the plurality of elongate thermal neutron guides extendto the external surface of the outer core.
 11. An instrument accordingto claim 1, wherein the plurality of elongate thermal neutron guidesextend to the inner core.
 12. An instrument according to claim 1,wherein the plurality of elongate thermal neutron guides contact theneutron detector.
 13. An instrument according to claim 1, wherein: (i)the plurality of elongate thermal neutron guides are substantially freeof the neutron-moderating material, and/or (ii) the plurality ofelongate thermal neutron guides are substantially free ofhydrogen-containing solid material, and/or (iii) the plurality ofelongate thermal neutron guides are substantially free ofboron-containing material, and/or (iv) the plurality of elongate thermalneutron guides extend in a radial direction from the inner core towardsthe external surface of the outer core, and/or (v) the plurality ofelongate thermal neutron guides have a line-of-sight along their length.14. An instrument according to claim 1, wherein the plurality ofelongate thermal neutron guides comprise or consist of a solid.
 15. Aninstrument according to claim 1, wherein the plurality of elongatethermal neutron guides comprise or consist of a metal.
 16. An instrumentaccording to claim 1, wherein the plurality of elongate thermal neutronguides comprise or consist of a metal selected from the group comprisingaluminum, copper, and lead.
 17. An instrument according to claim 1,wherein the plurality of elongate thermal neutron guides comprise afluid.
 18. An instrument according to claim 1, wherein the plurality ofelongate thermal neutron guides comprise or consist of air, a vacuum, ora partial vacuum.
 19. An instrument according to claim 1, wherein thetraverse (radial) cross-sectional areas of the plurality of elongatethermal neutron guides are substantially the same along their length.20. An instrument according to claim 1, wherein the traverse (radial)cross-sectional areas of the plurality of elongate thermal neutronguides are not the same along their length.
 21. An instrument accordingto claim 1, wherein the traverse (radial) cross-sectional areas of theplurality of elongate thermal neutron guides are greater for a portionof the guides located towards the external surface of the outer corethan for a portion of the guides located towards the inner core.
 22. Aninstrument according to claim 1, wherein the plurality of elongatethermal neutron guides terminate in a concave end.
 23. An instrumentaccording to claim 1, wherein the plurality of elongate thermal neutronguides include a protective terminal end, which prevents debris fromentering the guides.
 24. An instrument according to claim 23, whereinthe protective terminal end comprises a plug, or is a continuation ofthe wall(s) of the plurality of elongate thermal neutron guides, or is acontinuation of the outer core.
 25. An instrument according to claim 23,wherein the protective terminal end comprises a neutron-moderatingmaterial or a material transparent to thermal neutrons.
 26. A method ofdetecting radiation using an instrument according to claim 1, whereinsaid method comprises contacting the instrument with neutrons,generating one or more signals following contact between the neutronsand neutron detector, and detecting said one or more signals.
 27. Amethod according to claim 26, wherein said neutrons comprise thermalneutrons that are channeled towards the neutron detector by at least oneelongate thermal neutron guide.
 28. A method according to claim 26,wherein said neutrons comprise non-thermal neutrons.
 29. A methodaccording to claim 26, for enhancing the detection of thermal neutronsincident on an instrument that includes a neutron detector, said methodcomprising channeling thermal neutrons along at least one elongatethermal neutron guide, wherein the at least one elongate thermal neutronguide has an inner end positioned within 20 mm of the neutron detector,followed by detection thereof by the neutron detector.
 30. A methodaccording to claim 26, wherein the thermal neutrons have a mean energyof 25.3 meV at room temperature.
 31. A method according to claim 26,wherein the instrument simultaneously detects high energy neutrons.